When an internal combustion engine is started, if the intake manifold and/or intake air are cold, the low temperatures may make it difficult for fuel to vaporize in the engine cylinders. Any fuel that is incompletely vaporized may not be completely combusted. The portion of fuel that is not completely combusted at start-up and a period shortly after start-up may result in a fuel-rich exhaust mixture. Such a fuel-rich exhaust mixture may increase undesired hydrocarbon (HC) emissions and/or increased carbon monoxide levels in the exhaust.
For gasoline engines, engine heat production after startup may result in the incoming air gaining engine heat, prior to entering the engine cylinders. In other examples where the engine comprises a diesel engine, glow plugs may be employed to warm up engine cylinders upon initiation of a cold start event. However, for any vehicles equipped with stop/start (S/S) capability, where the engine may pull down (e.g. be deactivated to stop combusting air and fuel) when vehicle speed decreases to below a threshold speed, engine heat may not continue to rise monotonically, and thus at the next startup event, incoming air may not be sufficiently warmed. Such an issue may be exacerbated under conditions where ambient temperature is below a predetermined threshold, such as below freezing (e.g. <32° F.). Thus, in such an example, at a subsequent engine pull up (e.g. engine is activated to combust air and fuel), cold incoming air may result in combustion events that are not complete, thus resulting increased tailpipe emissions.
Various prior art devices have been employed to effect heat transfer to the intake air of an engine. For example, a manifold air heater system may help to raise the temperature of combustion air when intake manifold air passes through the intake manifold of the engine by means of an electrically heated element or a combustion burner using a liquid or gaseous fuel. However, the inventors have herein recognized issues with such an approach. For example, an intake manifold air heater system may add cost and complexity to a vehicle system. Furthermore, in the event that the heater is not functioning as desired, undesired emissions may result when a cold-start is initiated.
Accordingly, the inventors herein have developed systems and methods to at least partially address the above-mentioned issues. In one example, a method comprises spinning an engine of a vehicle in a reverse direction unfueled in response to an engine pull-down event until a temperature of an intake manifold of the engine rises to or above a threshold intake manifold temperature as a result of airflow from an exhaust manifold of the engine flowing through the engine and into the intake manifold. As an example, the threshold intake manifold temperature may comprise a temperature that results in a desired efficiency level of fuel combustion in response to a subsequent request to start the engine. In this way, fuel economy may be improved, and undesired emissions reduced.
In an example of the method, the engine pull-down event may comprise an S/S event that involves shut down of the engine to reduce an amount of time the engine spends idling. In some examples, spinning the engine in the reverse direction in response to the engine pull-down event may occur in response to a scheduled intake manifold heating operation. For example, scheduling the intake manifold heating operation may involve monitoring a color of an exhaust gas exiting an exhaust system of the engine at an engine start event, and responsive to an indication that the color of the exhaust gas is white, the intake manifold heating operation may be scheduled. In this way, based on an indication of poor fuel combustion at the engine start event, the intake manifold heating operation may be scheduled for subsequent engine pull-down events, so as to improve fuel combustion in response to engine start requests, which may improve fuel economy and reduce undesired emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.